One of the conventional ways of mounting components on a substrate is called surface mount technology (SMT). SMT components have terminals or leads (generally referred to as “electrical contacts”, “bumps”, or “pads”) that are soldered directly to the surface of a substrate. SMT components are widely used because of their compact size and simplicity of mounting. The electrical contacts of an SMT component are coupled to corresponding electrically conductive mounting or bonding pads (also referred to as “lands”) on the surface of the substrate, in order to establish secure physical and electrical connections between the component and the substrate. In order to fabricate PCBs in higher densities, it is known to surface-mount certain small passive components, such as capacitors, resistors, and inductors. The resulting electronic system can be manufactured at a lower cost and in a more compact size, and it is therefore more commercially attractive.
Before SMT components are mounted on a substrate, the substrate pads are selectively coated with corresponding solder deposits. Next, the component is carefully positioned or “registered” over the substrate, so that its electrical terminals are aligned with the corresponding substrate pads. Finally, in an operation known as “soldering,” the component terminals and the PCB pads are electrically and mechanically bonded together through a solidification of the solder deposits. An example of a soldering method includes solder reflow, a process during which the component terminals and the PCB pads are first heated to a temperature that melts the solder deposit, and during which the combination is then allowed to cool, so that the solider solidifies into solidified solder, and such that the terminals and pads thus make proper electrical and physical connections.
Typically, for example as seen in FIGS. 1a and 1b, a substrate 10 has pairs of pads 12 to which terminals 14 of SMT components, such as die side capacitor or DSC 16, can be mounted. Solder resist 15 is disposed between the two pads 12. Asymmetrical, lateral, surface-tension forces due to uneven surface tension of solder deposits 22 on the pads 12 during soldering can cause the DSC 16 to either shift, as seen in FIG. 1a, or tombstone, as seen in FIG. 1b. FIG. 1a shows a top view of DSC 16 as having shifted away from one of the substrate pads 12 to cover an adjacent substrate pad, while FIG. 1b shows a side view of DSC 16 as having tombstoned. Flipping, shifting and/or tombstoning of SMT components will be referred to herein as SMT component defects or SMTC defects. The tombstoning effect is considered a common soldering defect in the mounting of SMT components, and is caused by a combination of the surface tension of the solder, the SMT component's weight, and the soldering conditions. Another factor contributing to SMTC defects may include a vibration of the conveyor belt transporting the SMT component during soldering. SMTC defects having been observed at assembly sites especially recently with respect to DSC's whose dimension and weight have been reduced from 0805 (this terminology means that the components that have a length of 8 mil. and a width of 5 mil.) and 0402 to 0201. Because of the relatively small dimensions and weights of 0402 and 0201 components, the intricate balance of the surface tension may be more easily disturbed by either the change of the solderability of the components or by the differences of time at which the solder paste at each end of the component begins to melt.
The prior art has attempted to resolve SMTC defects caused during the mounting process by tuning either the solder paste printing process, the solder reflow process or the solder paste formulation. Tuning the solder paste printing process typically involves redesigning the printing stencils for the solder pads to change the solder printing parameters for reflow. Tuning the reflow process on the other hand typically involves extending the preheating time and the soaking time in order to achieve the desired balance between the surface tension forces on the component's terminals. A slower preheating rate has been shown to reduce SMTC defect rates. Tuning the paste formulation involves employing a solder alloy comprising tin/lead/silver in order to provide a wider solidification range and achieve balance between the surface tension of both side of a small leadless component. The expanded solidification range lengthens the higher tacky and pasty stage of the solder paste in the solder deposits, thus balancing a surface tension on the component's terminals, and in turn reduce the tombstoning frequency.
An alternative measure used in the prior art in order to reduce the occurrence of SMTC defects contemplates using an adhesive to hold the capacitor in place during soldering of a pre-mount combination 1 as shown. In such a method, as seen in FIG. 8, where like components are referred to using like reference numerals with respect to FIGS. 1a and 1b described above, an adhesive is dispensed on the solder resist 15 between the two substrate pads 12 as shown. The adhesive is meant to hold the capacitor in place during soldering in an attempt to reduce SMTC defects. However, disadvantageously, as SMT component sizes shrink, as noted in the paragraph above, use of the adhesive method becomes ill suited to combat SMTC defects to the extent that it among others requires an accurate placement of the adhesive and an accurate dispensing of the same, which become more difficult where small spaces/doses are involved, often requiring a fine tuning of the adhesive dispensing machine. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a significant need in the art for methods for mounting components to a substrate that offer relatively high density and high quality interconnections at a reasonable production cost.